Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on e.g. a silicon substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique for the next generation employing an exposure wavelength shorter than 45 nm, EUV lithography is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, EUV light is meant for light a ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for a light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional refracting optical system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a reflective optical system, i.e. a reflective photomask and reflective mirror, is employed.
A mask blank is a laminate before pattering, to be used for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass. As the reflective layer, it is common to use a Mo/Si multilayer reflective film having a molybdenum (Mo) layer as a high refractive index layer and a silicon (Si) layer as a low refractive index layer alternately laminated to have the light reflectivity improved when the layer surface is irradiated with EUV light.
For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is used.
Usually, a protective layer is formed between the above-described reflective layer and the absorber layer. Such a protective layer is one to be provided for the purpose of protecting the reflective layer, so that the reflective layer will not be damaged by an etching process to be carried out for the purpose of forming a pattern on the absorber layer. In Patent Document 1, it is proposed to use ruthenium (Ru) as the material for the protective layer. In Patent Document 2, a protective layer is proposed which is made of a ruthenium compound (Ru content: 10 to 95 at %) containing Ru and at least one member selected from Mo, Nb, Zr, Y, B, Ti and La.
A mirror to be used for EUV lithography has a structure wherein a reflective layer for reflecting EUV light is formed on a substrate made of e.g. glass. As the reflective layer, a multilayer reflective film having high refractive index layers and low refractive index layers alternately laminated plural times is usually used, since a high EUV light reflectivity can be thereby accomplished. Therefore, as a mirror to be used for EUV lithography, such a multilayer mirror having a multilayer reflective film formed on a substrate is usually employed (Patent Document 3).
In such a multilayer mirror, a protective layer (a protective capping layer) is formed on such a multilayer reflective film for the purpose of protecting the multilayer reflective film from chemical or physical erosion in many cases. Patent Document 3 discloses that an EUV mirror is configured such that a specific capping layer (protective layer) is formed on a reflective layer for the purpose of having resistance to chemical or physical erosion. The multilayer mirror disclosed in Patent Document 3 is provided with a protective capping layer made of a material selected from ruthenium (Ru) and rhodium (Rh), and their compounds and alloys.